In communication systems incorporating mobile radio receivers and transmitters (transceivers) a problem occurs because of the Doppler Shift in the perceived frequency of the received signals created by the movement of the receiver. Radio receivers and transmitters utilise local oscillators to provide a reference signal of a particular frequency for modulating or demodulating the transmission signals. Another problem occurs in connection with this in that there will be differences in the frequency of each of the oscillators.
It is known in radio-transmitter and receiver apparatus to provide a circuit which monitors an output of a local oscillator to determine a frequency offset or error. This error is fed back to the local oscillator to correct the frequency. Such circuits are commonly known as automatic frequency control (AFC) circuits. Another cause of frequency deviation in the transmitted signal is Rayleigh fading. This causes large nearly instantaneous or momentary deviations in the perceived frequency error.